Due to advances in materials and manufacturing process, fabricating components using powder metallurgy (“P/M”) techniques is becoming increasingly more widespread as an alternative to other metalworking technologies. P/M provides the capability of forming high quality components to close tolerances in an economical fashion. Since P/M techniques can be easily automated to produce net-shape or near net-shape components having complex shapes, fabrication by P/M techniques often increases cost savings, precision, productivity, and materials conservation.
Although P/M encompasses many fabrication techniques, these techniques typically include the processes of forming a “green” part and sintering. A green part is a structure that can be formed by a variety of methods, including by compacting a powder in a die and by mixing a powder with a binder that allows the mixture to be injection molded in a manner similar to injection molding of plastics. Although the green part resulting from compaction or injection molding has a well-defined geometric shape, it lacks desired properties, such as, for example, strength, toughness, and porosity, of the final component. The desired properties are produced by a heating process, known as sintering. The green part is sintered at a temperature below the melting temperature of the main constituent of the powder to generate metallurgical bonds between the powder particles without distorting the shape of the component. Once sintering is completed, the component can be further treated, if necessary, by secondary operations, such as, for example, oil impregnation, resin impregnation, dipping, machining, and assembly.
Assembling P/M components presents unique challenges when the P/M components must be joined to one another or to non-P/M materials, such as, for example, wrought or cast materials. Brazing is a group of processes that uses a filler metal to join components. The filler metal typically has a melting temperature above 450° C., but below the melting temperature of the base materials of the components to be joined. As the filler metal is heated, it is drawn into the joint between the components by capillary action. P/M components can be joined by brazing either after sintering or during the sintering process. Due to the porosity of the P/M component, however, the capillary action that draws the molten filler metal into the joint also draws the molten filler material away from the joint and into the pores of the P/M component. This results in excessive infiltration and poor joint quality.
A powder filler metal disclosed in U.S. Pat. No. 3,717,442 attempts to overcome the problem of excessive infiltration into P/M components. This filler metal is, however, brittle and difficult to cold work into useful preform shapes. As a result, preforms of this braze powder must be of substantial bulk to withstand handling prior to brazing. In joints where excess filler metal compromises the quality of the assembly, such as, for example, weeping into splines, slugs of this filler metal can be formed and placed at precise intervals in the joint. These slugs, however, can cause intermittent and poor quality joints if braze flow is erratic. Additionally, placing the slugs at the correct intervals is more labor intensive than the use of a preform shape, such as, for example, a braze ring.
Thus, there is a need to overcome these and other problems of the prior art and to provide a braze preform suitable for brazing where at least one of the components to be joined is a P/M component. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above.